Field of the Invention
This invention relates to a signal generator which is capable of providing a frequency signal as requested by digitally processing a given reference signal from a reference signal generator such as a crystal-controlled oscillator.
There has been hitherto known a signal generator which is capable of providing as low a frequency as requested by digitally processing a clock signal from a clock signal generator such as a crystal controlled oscillator, and such frequency-selectable signal generator have been widely used in different fields of art. On typical example of the use of such frequency-selectable signal generator is found in an electronic musical instrument. A conventional electronic instrument uses a "period counting system" in which each of different-notes of the musical scale is produced by counting and picking up clock pulses at a predetermined interval corresponding to the frequency of the note. The accuracy with which the "period counting system" can produce notes depends on the resolving power of the clock signal, and therefore a clock signal of as high a frequency as 4 MHz is used in the hope of producing fairly accurate musical notes. Recently, a small-sized electronic device such as an electronic watch, having a musical function has been available in the market. As is well known, such a small-sized electronic device uses a minute button type cell as a power supply, and therefore, a 4 MHz oscillator cannot be used as a clock signal generator because of the shortening of the life of the cell which would be caused by the use of the 4 MHz oscillator. In this connection and in view of cost and space saving a crystal-controlled oscillator of 32,768 Hz is commonly used as a reference signal generator and, at the same time, in an electronic watch as a clock signal generator for providing notes of the musical scale. Then, the melody-producing electronic watch uses a "period counting system" which is designed to work with clock signal of as low a frequency as 32,768 Hz. The use of such low frequency clock signal inevitably deteriorates the quality of melody because of inaccuracy in resultant notes of the musical scale, as for instance 885.6 Hz or 862.3 Hz are obtained for Note "A" or 880 Hz as a division of 32,768 Hz by 37 or 38 respectively. In an attempt to reduce such offset from the exact notes, a frequency multiplying circuit is used at the cost of increase of consumption of electric power and complicated structure of electrical circuits. Thus, the reference frequency is increased to 65,536 Hz, and then the high frequency is divided to obtain notes of musical scale as requested. The result is halving of the error which otherwise, would appear in resultant notes of the musical scale.
In contrast to this, there has been proposed a system for digitally processing clock signals with a higher accuracy than the "period counting system". For example, in reducing 32,768 Hz to 10 Hz, which low frequency is inevitably used in a stopwatch mode, 32,768 Hz is divided by the "n"th of 2, and 128 Hz thus obtained is divided alternately by 12 and by 13. This proposed system has satisfactorily met the demand for reducing a high frequency to a single low frequency. It, however, cannot meet the demand for reducing a high frequency to a plurality of different frequencies corresponding to the particular notes of the musical scale for the following reasons: Two different ratios of division are set for each note of the musical scale, and these different ratios are selectively used to produce a particular note. This necessitates a control circuit appropriate for the purpose. The control circuit, however, is inhibitably complicated in structure, and indeed, an electronic watch cannot be equipped with such control circuit.
Also, there has been proposed a system commonly called "Rate-multiplier", which is capable of providing directly a frequency signal as requested. This proposed system, however, has a disadvantage of producing note signals in the form of pulses at irregular intervals. Therefore, a melody cannot be made with recourse to the rate-multiplier. This is described in detail with reference to FIGS. 1-3 as follows:
FIG. 1 shows a conventional rate-multiplier as including a frequency divider 1 for dividing a clock signal "CL". The frequency divider 1 is composed of a series-arrangement of six flip-flops "1a" to "1f", each producing an output signal at its terminal "Q". These output signals are directed to as many AND gates "3a" to "3f" directly or via associated inverters "2a" to "2f". Each AND gate is supplied with an inverted output signal from an associated flip-flop and output signals at higher frequencies from the antescedent flip-flops, producing an output of logical multiplication of these input signals. For instance, an output signal from the final stage "1a" is inverted by an inverter "2a" , and then directed to the AND gate "3a" whereas the output signals from the other stages "1b" to "1f" are directed to the AND gate "3a" without being inverted. The output signals S0-S5 from the AND gates "3a" to "3f" are directed to an OR gate, and an output signal appears at the terminal OUT 1 of the rate-multiplier. As seen from FIG. 1, control signals in the form of binary-coded parallel data corresponding to a musical note as demanded are supplied over conductors "D0" to "D5" to put the AND gates selectively open, thereby causing the rate-multiplier to generate the output signal at the same frequency as the musical note as its output terminal OUT 1.
FIG. 2 shows waveforms of different output signals S0-S5 appearing at the output terminal of the AND gates "3a" to "3f". In a 6-bit type rate-multiplier as shown in FIG. 1, six different trains of pulses, not occurring concurrently, appear, and 64 different output signals can be obtained by combining those six elemental trains of pulses.
FIG. 3 shows 12 different waveforms of signals appearing at the output terminal OUT 1 of the rate-multiplier within one cycle which is a period equivalent to 64 clock pulses. Sixty-four waveforms 0 to 63 can be produced with binary-coded parallel numeric data. As is apparent from the graphic representation, pulses appear at regular intervals for parallel numeric data representing numbers 2,4,8, . . . (2.sup.n in general) whereas pulses appear at irregular intervals for the remainder parallel data, as for instance "16-16-16-8-8" for parallel datum representing number 5 and "16-8-8-8-8-8-8" for parallel datum representing number 7. These repetitive patterns, however undesirably large in pulse-to-pulse irregularity, compared with those which could be obtained in a digital processing system ("13-13-13-13-12" for parallel datum representing a decimal number 5 and "10-9-9-9-9-9-9" for parallel datum representing number 7). As for parallel datum representing a decimal number 5, two different intervals appear two or more times in succession within one cyclic period as is "16-16-16-8-8". For this reason, the sound thus produced is unpleasing to ear, and in fact, cannot be used to make a musical melody.